Oil extraction of oilseeds such as rapeseeds or sunflower seeds is an energy intensive process as it involves several steps requiring mechanical and thermal energy. This energy is partially mechanical, e.g., breaking, grinding, rolling pressing and pelletizing, and partially thermal to degrade cell walls, reduce oil viscosity and adjust moisture content of any intermediate or final product of the process. Before oil extraction proper, the oilseeds must be prepared: energy must be used to rupture or weaken the walls of the oil-containing cells. For some seeds, for example sunflower, a dehulling is also recommended.
The oilseed crushing process will be described in more detail for rapeseed which is one of the major oilseeds with a worldwide production exceeding 62 million tons in 2009 and steadily increasing. The oilseed crushing process is broken down in several steps: cleaning, preheating, flaking, cooking, prepressing and solvent extraction. During the cleaning step, dust, oversized and foreign particles are removed from the oilseeds. During the preheating step, the seeds are preheated from ambient temperature to about 50-70° C. by indirect heating or direct hot air contact. This preheating weakens the cell structure and softens the rapeseed which improves subsequent flaking. During the flaking step, the preheated rapeseeds are flaked between steel rolls with the aim to reduce the thickness which in the downstream solvent extraction step allows the solvent, usually hexane, to penetrate into the cellular structure to dissolve and extract the oil. Before solvent extraction, the flakes must be cooked, typically in a stacked vertical steam-tray cooker or in a horizontal rotary steam-tube cooker to about 80-100° C. In this cooking step the flakes are not only heated but the moisture content is reduced from about 8-10% to about 4-6%. Therefore, the cooking of the flakes generates a hot vapour stream. This vapour stream is contaminated by fatty material, fine particles of seed (fines) and often odoriferous components. The flakes are finally prepressed and solvent extracted to yield rapeseed oil and rapeseed meal that both need further processing. Typically, the oil is refined by several successive steps, and the meal is desolventised. After desolventisation, the rapeseed meal is then dried in specific equipment such as for example a vertical stacked fluidized air tray dryer or a horizontal rotary steam-tube dryer. The meal dryer typically reduces the meal from approximately 14-18% moisture to 10-12% moisture. Therefore, a second hot vapour stream, usually contaminated by fine meal particles (fines) and often odoriferous components is emitted from the meal drying step.
Accordingly, as illustrated above, the oilseed crushing process is energy intensive with a substantial thermal heating step and two substantial thermal drying steps, creating the potential for energy savings.
An efficient energy recovery solution is described in EU-project LIFE04 env/d/000051. In this energy recovery process, the thermal energy contained in the hot vapours leaving the flake cooking step is valorised to preheat the rapeseed in the seed preheating step. This recovery process involves the scrubbing of the exhaust hot vapours leaving the flake cooking step to generate hot water which is then used to preheat the rapeseed entering the crushing process through the use of a preheater comprised of vertical heat exchanger plates. The rapeseeds are flowing between the plates by gravity and are preheated by conductivity. This process is indeed efficient to recover thermal energy contained in the hot vapours leaving the flake cooking step; however, the scrubbing of those hot vapours generates hot water contaminated by fatty material and fines. Therefore, the cleaning and the maintenance of the equipment in contact with the hot water contaminated by fatty material and fines are difficult. Alternatively, the hot vapours stream(s) could be cleaned by filter media or cyclonic separation but those methods are inefficient since they lead to a rapid clogging of the cleaning means due to the sticky nature of the contaminants created by the protein contained in the fines.
It is therefore the aim of the present invention to describe a process to recover thermal energy contained in the hot vapour stream(s) contaminated by fatty material and/or fines and/or odoriferous components leaving the flake cooking step or the meal drying step of an oilseed crushing process, with such process incurring minimum fouling of heat transfer mechanisms to maintain continuous high efficiency with minimum cleaning or maintenance.